As someone said, LARGER sized pens or markers work best. You can find Disney-themed markers about everywhere now. On our last trip, we had Tinkerbell markers .. a pink one for my younger daughter and a purple for my older daughter. That way we were able to easily identify which book was which just at a glance.
Also, we don't use little flimsy autograph books. You can often find sturdier spiral bound small scrapbooks approximately the same size as an autograph book at places like Michaels or scrapbook stores. The beauty of these is that the paper stock is much heavier and you don't have to worry about ink bleeding through. (So, if you want to use those large/fat sharpies, you can with NO problem) Additionally, we make a big deal about then scrapping the pages after the trip ... we try to leave the page across from an autograph blank for journaling and a pic.
Also ... if you are getting autographs, just observing common courtesy will help the line move along ... have the book out & open to the page ... have your pen ready (IE ..if the cap needs to be removed, do that BEFORE you get up there) ... have your camera set & ready to go.
If you observe a little plain ol' common sense, there is no reason EVERYONE who enjoys meeting characters can't do so ... regardless of your age or the age of whoever is in line behind you. Sometimes, only our kids meet characters, but other times we like to get a family picture for our scrapbook.
Since I love Disney as much if not more that my children, I have absolutely no hesitation about enjoying whatever aspects I want to .... even if others believe their should be an age limit on some things.
To whoever posted the Walt quote about a trip to WDW being family time & adults being kids at heart ... all the young-at-heart adults who love to meet characters thank you!!!!